1. Field of the Invention
This invention relates to an apparatus for evaluating the state of a polysilicon film generated on annealing amorphous silicon.
2. Description of Related Art
Recently, a thin-film transistor, employing a polysilicon film as its channel layer, is being put to practical use. Such the thin-film transistor employing a polysilicon film is appreciably improved in electrical field mobility, so that electric devices e.g., liquid crystal display device, using the thin-film transistor as driving circuit devices, can be improved significantly in definition, operating speed and miniaturization.
On the other hand, development of the so-called low-temperature poly-crystallization process, in which amorphous silicon is heat-treated to form a polysilicon film using a excimer laser annealing device, is also under way. By applying this low-temperature poly-crystallization process as the manufacturing process for the thin-film transistor, thermal damage to the glass substrate may be diminished to permit the use of an inexpensive large-area heat-resistant glass substrate.
However, the excimer laser annealing device, used in the low-temperature poly-crystallization process, is unstable in laser output power, with the result that the grain size of the polysilicon film formed is fluctuated significantly. The result is that the polysilicon film, prepared using this excimer laser annealing device, is not necessarily of the optimum grain size, thus occasionally being discarded as a reject.
So, the conventional practice in carrying out the annealing using this excimer laser annealing device is to perform 100% inspection or sampling inspection, at a stage of completion of the poly-crystallization of the polysilicon film, and to check for the state of crystals of the polysilicon film formed on the uppermost surface of the polysilicon film, thereby to decide whether or not the products are acceptable at this stage. Additionally, the information on the energy afforded from the excimer laser annealing device to the polysilicon film is fed back to the excimer laser annealing device to set an optimum laser power.
However, there has not been a method which objectively evaluates the polysilicon film in non-contact fashion, while there has been a subjective method such as a sensual method of photographing a surface image, using a microscope for visible light or a scanning electron microscope, to visually inspect the surface image to check for the crystal state by a operator. In addition, this known method has other problems that it can not be used as an in-process method, because it is costly and time-consuming. Among other known methods, the spectroscopic evaluation method by ellipsography is not satisfactory as to its quantitative characteristics.
In view of the above depicted status of the art, it is an object of the present invention to provide a polysilicon film evaluating apparatus whereby the status of the as-formed polysilicon film can be evaluated to a high accuracy objectively automatically in a non-contact fashion.
The present invention provides an apparatus for evaluating a polysilicon film formed on annealing an amorphous silicon film, including a stage for setting a substrate thereon, the substrate carrying a polysilicon film formed thereon, an optical system for observation with the visible light, the optical system for observation with the visible light illuminating the visible light on the substrate on the stage for photographing a surface image of the polysilicon film on the substrate to effect auto-focusing, an optical system for observation with UV light, illuminating the UV light on the substrate on the stage for acquiring a surface image of the polysilicon film on the substrate, auto-focused using the optical system for observation with the visible light, and evaluation means for evaluating the linearity and periodicity of a spatial structure of the film surface of the polysilicon film from the surface image of the polysilicon film acquired by the optical system for observation with UV light to evaluate the state of the polysilicon film based on the results of evaluation of the linearity and periodicity.
With the apparatus for evaluating a polysilicon film, according to the present invention, the status of the as-formed polysilicon film can be evaluated to a high accuracy objectively automatically in a non-contact fashion.